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  <div id="app">发布订阅模式：模拟自定义事件实现机制</div>

  <script>
    // 事件触发器
    class EventEmitter {

      //   定义构造函数
      constructor() {
        //   {'click':[fn1, fn2], 'change': [fn1]}
        this.subs = Object.create(null)
      }

      // 注册事件
      $on(eventType, handler) {
        this.subs[eventType] = this.subs[eventType] || []
        this.subs[eventType].push(handler)
      }

      // 触发事件
      $emit(eventType) {
        if (this.subs[eventType]) {
          this.subs[eventType].forEach(handler => {
            handler()
          })
        }
      }
    }

    // 测试
    let em = new EventEmitter()

    em.$on('click', () => {
      console.log('click-1.')
    })

    em.$on('click', () => {
      console.log('click-2.')
    })

    em.$emit('click')

  </script>

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